Description:

Organic aerosol is an important component of ambient particulate matter, whether in urban, rural, or remote areas. Organic compounds represent on average 30% of the PM10 and 35% of the PM2.5 aerosol mass in urban polluted areas like Los Angeles. Atmospheric aerosols pose significant environmental risks (health problems, visibility degradation), in part because of these organic compounds. Design of effective emission controls, that will reduce the ambient aerosol concentrations and the corresponding risks, require the understanding of the processes leading to the formation of organic aerosols. The direct emission of organic matter to the atmosphere (primary organic aerosol) is a relatively simple process, and current efforts mainly concentrate on the quantification of emission rates and the chemical characterization of sources. On the contrary, the in situ formation of secondary organic aerosol (SOA) by condensation of low volatility products of the photooxidation of hydrocarbons remains poorly understood. The available aerosol thermodynamic models often cannot explain the reported presence of several semi-volatile organic compounds in the aerosol phase. The state-of-the-art dynamic aerosol models cannot reproduce the observed SOA bimodal distribution. Current understanding of the interaction of these SOA compounds with each other and with the inorganic aerosol components is very limited.

This project investigates the transport of SOA compounds from the gas to the aerosol phase. The equilibrium partitioning of the major SOA components between the gas and aerosol phases is studied by measuring the vapor pressures of their mixtures. The condensation rates of these SOA compounds on a variety of model inorganic and organic aerosols will be measured and the corresponding accommodation coefficients will be calculated. The hypothesis that the SOA compounds preferentially condense on particles of a given composition will be tested. The effect of these organic compounds in the evaporation rate of NH4NO3, the major volatile inorganic aerosol component, will be determined. The proposed experimental system is based on the Tandem Differential Mobility Analyzer (TDMA) technique. A TDMA system includes the recently developed Scanning Electrical Mobility Spectrometer (SEMS) allowing detailed measurements of the fine aerosol size distribution in a timescale of seconds and the additional ability to produce an externally mixed monodisperse aerosol population. The measured variables will be incorporated in a state-of-the-art aerosol model (Size Resolved Secondary Organic Aerosol Model, SRSOAM). The ability to model the SOA formation processes in the ambient atmosphere will be evaluated by comparing the model output with the best available urban air quality data set (Southern California Air Quality Study, 1987).

Progress and Final Reports:

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.